In recent years, notebook-sized personal computers, cellular phones, PDAs (Personal Digital Assistant) and other portable terminals have been remarkably widespread. As batteries for the portable terminals, a lithium ion secondary battery (hereinafter, also simply referred to as a battery) is often used. Due to demands for more comfortable portability, the portable terminals have been rapidly made more compact, thinner, lighter, and higher in performance and become used in various occasions. This movement has been still continuing today and there are also demands for batteries to be more compact, thinner, lighter, and higher in performance.
A lithium ion secondary battery has a structure of containing in a container a positive electrode and a negative electrode arranged over a separator together with an electrolyte solution wherein lithium salt, such as LiPF6 and LiBF4, is dissolved in an organic liquid of ethylene carbonate or the like. The positive electrode of the secondary battery is obtained by mutually binding LiCoO2, LiMn2O4, or the like as an active material and acetylene black, or the like as a conductive material by a secondary battery electrode binder (hereinafter, also simply referred to as a binder) and further binding the result to aluminum, etc. as a collector. The negative electrode of the secondary battery is obtained by mutually binding by a binder a carbonaceous material as an active material and the same conductive material as above, etc. in accordance with need, and further binding the result to a collector of copper, etc.
The positive and negative electrodes are usually formed by dissolving or dispersing a binder for an electrode in a liquid medium, applying an electrode slurry composition obtained by mixing the result with an active material, conductivity adding agent, etc. (hereinafter, also simply referred to as a slurry) to a collector, removing the liquid medium by drying, etc. and binding as a mixed layer.
Recently, there are stronger demands for a longer use time of portable terminals and a shorter charging time thereof, so attaining of a larger capacity and an improvement of charging speed (a rate characteristic) of batteries are urgent. A battery capacity is strongly affected by an amount of an active material and the rate characteristic is affected by easiness of electron movement. Suppression of a binder amount is effective for increasing an active material in a limited space of a battery, while it is limited because a binding property of the active material is deteriorated when the binder amount is reduced. Also, since the binder is a non-conductive polymer and is liable to hinder electron movement, there has been an attempt for improving the rate characteristic by adding a conductivity adding agent, such as carbon. However, adding of a conductivity adding agent results in limiting a use amount of an active material and an improvement of a battery capacity is hardly expected.
As explained above, attaining of a battery having a larger capacity has been thus far hard to consist with an improvement of the rate characteristic.